The present invention relates to a RF receiver coil arrangement for NMR spectrometers comprising a RF receiver coil which is arranged around a sample and cooled down to far below room temperature, for receiving nuclear magnetic resonance signals from the sample, the latter being positioned in a homogeneous magnetic field and being adjusted substantially to room temperature or to a higher temperature.
Such a spectrometer RF receiver coil arrangement has been known from a paper by Hoult et al. published in Journal of Magnetic Resonance 24, 71-85 (1976). In the case of the arrangement described by this publication, the entire RF receiver coil is accommodated in a Dewar, which is cooled by means of a cryogenic liquid, and is isolated from the sample by a vacuum in a double-walled glass vessel. The main drawback of this conventional arrangement consists in the fact that the spacing between the sample and the RF receiver coil must be equal to at least the thickness of two glass walls plus the latter's spacing which contains the vacuum, which results in a significant reduction in space factor. Another disadvantage is seen in the system's the sensitiveness to position, and finally gas bubbles rising in the cooling liquid may also interfere with the measurement.
An improved cooled RF receiver coil arrangement for nuclear magnetic resonance spectrometers has been known from publications by Styles et al. (Journal of Magnetic Resonance 60, 397-404 (1984) and 84, 376-378 (1989)). In the case of this design, the RF receiver coil is positioned in a vacuum, is designed as a hollow tube, and is passed by a cryogenic liquid and is, thus, cooled from its inside. Compared with the before-mentioned conventional structure using a double-walled glass Dewar, one of these walls can be omitted when the RF receiver coil is placed in the vacuum. However, it is a substantial drawback of this arrangement that it restricts considerably the constructional freedom with respect to the RF receiver coil. The very limited choice of materials available for use makes it difficult to provide for the desired susceptibility compensation of the RF receiver coil, and results in certain losses as regards resolution and detection efficiency. In addition, the production of such a coil is difficult, and the design options for the coil are rather limited due to the fact that the tube forming the coil cannot be produced in any shape, and cannot be deformed at desire, either. This again has the result that it is not possible to achieve the best possible space factor. In addition, exchanging the RF receiver coil is rendered difficult, due to the necessary soldered joints. In cooled operation, the cryogenic agent which is forced through the tube under pressure may give rise to vibrations in the RF receiver coil which may result in susceptibility variations, a phenomenon which reduces the quality of the measurement still further. Finally, it has been noted in both of the before-mentioned publications that experiments conducted with the arrangement with internally cooled RF receiver coil led to only about half the improvement of the signal-to-noise ratio that had been expected theoretically.